This invention relates in general to optical components used in optical communication, and in particular a polarization beam combiner for fiber optic applications.
In an effort to keep pace with bandwidth demands nearly doubling annually, Wavelength Division Multiplex (WDM) system designers are moving to more cost effective solutions utilizing higher channel counts, higher transmission speeds and longer distance. This move drives up the need for more power in optical fibers.
The Erbium-Doped Fiber Amplifiers (EDFA) and Raman Amplifiers are two of most popular devices for amplifying the optical signal along the fiber. Diode pump lasers in 980 nm, 1480 nm and 14xx nm are used for power pumping. The optical power from current commercial diodes can go up to 250 mW. But, much more power is needed for WDM Amplifiers, particularly for Raman Amplifiers.
One conventional solution as indicated in FIG. 1 for more power is to combine multiple pump lasers supplying light at different wavelengths and in different polarization states before launching into the transmission fiber. The polarization beam combiner (PBC) is used to combine two laser powers in the same wavelength as well as different wavelengths, but in orthogonal states. For more gain, it is desirable that the PBC has low loss. A pump multiplexer is then used to combine all laser powers in different wavelengths.
FIGS. 2A and 2B are two prior art PBCs. In FIG. 2A, multiple layers of dielectric material at the diagonal interface between the two prisms are utilized to combine two beams traveling at right angles to each other. A birefringent crystal in FIG. 2B is used to combine two orthogonally polarized beams but directed through two separate GRIN lenses apart from each other. The prior art PBCs of both FIG. 1 and of FIGS. 2A and 2B are bulky because the fibers are either oriented at right angles to one another or are transmitted using separate GRIN lenses.
It is therefore desirable to provide a low cost and easy-to install Polarization Beam Combiner that is more compact than the conventional PBCs.
This invention is based on the observation that, by employing the same GRIN lens to pass two input optical beams, where the two input beams are at different radial distances relative to the lens, a more compact PBC can be achieved. Since the two input optical beams are at different radial distances relative to the lens, the input beams will exit the lens at different angles. An optical member having transverse optical axes are then used to pass and combine the two beams exiting the lens, wherein the member has different indices of refraction along the two axes, thereby combining the two beams into one along an output channel.